Zorn Microsoft Corporation July 1999 Point-to-Point Tunneling Protocol (PPTP) Status of this Memo This memo provides information for the Internet community

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Category: Informational G. Pall Microsoft Corporation W. Verthein 3Com J. Taarud Copper Mountain Networks W. Little ECI Telematics G. Zorn Microsoft Corporation July 1999

Point-to-Point Tunneling Protocol (PPTP) Status of this Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

IESG Note

The PPTP protocol was developed by a vendor consortium. The documentation of PPTP is provided as information to the Internet community. The PPP WG is currently defining a Standards Track protocol (L2TP) for tunneling PPP across packet-switched networks.

Abstract

This document specifies a protocol which allows the Point to Point Protocol (PPP) to be tunneled through an IP network. PPTP does not specify any changes to the PPP protocol but rather describes a new vehicle for carrying PPP. A client-server architecture is defined in order to decouple functions which exist in current Network Access Servers (NAS) and support Virtual Private Networks (VPNs). The PPTP Network Server (PNS) is envisioned to run on a general purpose

operating system while the client, referred to as a PPTP Access Concentrator (PAC) operates on a dial access platform. PPTP specifies a call-control and management protocol which allows the server to control access for dial-in circuit switched calls

originating from a PSTN or ISDN or to initiate outbound circuit-

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switched connections. PPTP uses an enhanced GRE (Generic Routing Encapsulation) mechanism to provide a flow- and congestion-controlled encapsulated datagram service for carrying PPP packets.

Specification of Requirements

In this document, the key words "MAY", "MUST, "MUST NOT", "optional", "recommended", "SHOULD", and "SHOULD NOT" are to be interpreted as described in [12].

The words "silently discard", when used in reference to the behavior of an implementation upon receipt of an incoming packet, are to be interpreted as follows: the implementation discards the datagram without further processing, and without indicating an error to the sender. The implementation SHOULD provide the capability of logging the error, including the contents of the discarded datagram, and SHOULD record the event in a statistics counter.

Table of Contents

1. Introduction . . . 3

1.1. Protocol Goals and Assumptions . . . 4

1.2. Terminology . . . 5

1.3. Protocol Overview . . . 6

1.3.1. Control Connection Overview . . . 7

1.3.2. Tunnel Protocol Overview . . . 7

1.4. Message Format and Protocol Extensibility . . . 8

2. Control Connection Protocol Specification . . . 10

2.1. Start-Control-Connection-Request . . . 10

2.2. Start-Control-Connection-Reply . . . 12

2.3. Stop-Control-Connection-Request . . . 15

2.4. Stop-Control-Connection-Reply . . . 16

2.5. Echo-Request . . . 17

2.6. Echo-Reply . . . 18

2.7. Outgoing-Call-Request . . . 19

2.8. Outgoing-Call-Reply . . . 22

2.9. Incoming-Call-Request . . . 25

2.10. Incoming-Call-Reply . . . 28

2.11. Incoming-Call-Connected . . . 29

2.12. Call-Clear-Request . . . 31

2.13. Call-Disconnect-Notify . . . 32

2.14. WAN-Error-Notify . . . 33

2.15. Set-Link-Info . . . 35

2.16. General Error Codes . . . 36

3. Control Connection Protocol Operation . . . 36

3.1. Control Connection States . . . 37

3.1.1. Control Connection Originator (may be PAC or PNS) . . . 37

3.1.2. Control connection Receiver (may be PAC or PNS) . . . . 39

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3.1.3. Start Control Connection Initiation Request Collision . 40

3.1.4. Keep Alives and Timers . . . 40

3.2. Call States . . . 41

3.2.1. Timing considerations . . . 41

3.2.2. Call ID Values . . . 41

3.2.3. Incoming Calls . . . 41

3.2.3.1. PAC Incoming Call States . . . 42

3.2.3.2. PNS Incoming Call States . . . 43

3.2.4. Outgoing Calls . . . 44

3.2.4.1. PAC Outgoing Call States . . . 45

3.2.4.2. PNS Outgoing Call States . . . 46

4. Tunnel Protocol Operation . . . 47

4.1. Enhanced GRE header . . . 47

4.2. Sliding Window Protocol . . . 49

4.2.1. Initial Window Size . . . 49

4.2.2. Closing the Window . . . 49

4.2.3. Opening the Window . . . 50

4.2.4. Window Overflow . . . 50

4.2.5. Multi-packet Acknowledgment . . . 50

4.3. Out-of-sequence Packets . . . 50

4.4. Acknowledgment Time-Outs . . . 51

4.4.1. Calculating Adaptive Acknowledgment Time-Out . . . 53

4.4.2. Congestion Control: Adjusting for Time-Out . . . 54

5. Security Considerations . . . 54

6. Authors’ Addresses . . . 55

7. References . . . 56

8. Full Copyright Statement . . . 57 1. Introduction

PPTP allows existing Network Access Server (NAS) functions to be separated using a client-server architecture. Traditionally, the following functions are implemented by a NAS:

1) Physical native interfacing to PSTN or ISDN and control of external modems or terminal adapters.

A NAS may interface directly to a telco analog or digital circuit or attach via an external modem or terminal adapter.

Control of a circuit-switched connection is accomplished with either modem control or DSS1 ISDN call control protocols.

The NAS, in conjunction with the modem or terminal adapters, may perform rate adaption, analog to digital conversion, sync to async conversion or a number of other alterations of data streams.

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2) Logical termination of a Point-to-Point-Protocol (PPP) Link Control Protocol (LCP) session.

3) Participation in PPP authentication protocols [3,9,10].

4) Channel aggregation and bundle management for PPP Multilink Protocol.

5) Logical termination of various PPP network control protocols (NCP).

6) Multiprotocol routing and bridging between NAS interfaces.

PPTP divides these functions between the PAC and PNS. The PAC is responsible for functions 1, 2, and possibly 3. The PNS may be

responsible for function 3 and is responsible for functions 4, 5, and 6. The protocol used to carry PPP protocol data units (PDUs) between the PAC and PNS, as well as call control and management is addressed by PPTP.

The decoupling of NAS functions offers these benefits:

Flexible IP address management. Dial-in users may maintain a

single IP address as they dial into different PACs as long as they are served from a common PNS. If an enterprise network uses

unregistered addresses, a PNS associated with the enterprise assigns addresses meaningful to the private network.

Support of non-IP protocols for dial networks behind IP networks.

This allows Appletalk and IPX, for example to be tunneled through an IP-only provider. The PAC need not be capable of processing these protocols.

A solution to the "multilink hunt-group splitting" problem.

Multilink PPP, typically used to aggregate ISDN B channels,

requires that all of the channels composing a multilink bundle be grouped at a single NAS. Since a multilink PPP bundle can be handled by a single PNS, the channels comprising the bundle may be spread across multiple PACs.

1.1. Protocol Goals and Assumptions

The PPTP protocol is implemented only by the PAC and PNS. No other systems need to be aware of PPTP. Dial networks may be connected to a PAC without being aware of PPTP. Standard PPP client software should continue to operate on tunneled PPP links.

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PPTP can also be used to tunnel a PPP session over an IP network. In this configuration the PPTP tunnel and the PPP session runs between the same two machines with the caller acting as a PNS.

It is envisioned that there will be a many-to-many relationship between PACs and PNSs. A PAC may provide service to many PNSs. For example, an Internet service provider may choose to support PPTP for a number of private network clients and create VPNs for them. Each private network may operate one or more PNSs. A single PNS may associate with many PACs to concentrate traffic from a large number of geographically diverse sites.

PPTP uses an extended version of GRE to carry user PPP packets. These enhancements allow for low-level congestion and flow control to be provided on the tunnels used to carry user data between PAC and PNS.

This mechanism allows for efficient use of the bandwidth available for the tunnels and avoids unnecessary retransmisions and buffer overruns. PPTP does not dictate the particular algorithms to be used for this low level control but it does define the parameters that must be communicated in order to allow such algorithms to work.

Suggested algorithms are included in section 4.

1.2. Terminology Analog Channel

A circuit-switched communication path which is intended to carry 3.1 Khz audio in each direction.

Digital Channel

A circuit-switched communication path which is intended to carry digital information in each direction.

Call

A connection or attempted connection between two terminal endpoints on a PSTN or ISDN -- for example, a telephone call between two modems.

Control Connection

A control connection is created for each PAC, PNS pair and

operates over TCP [4]. The control connection governs aspects of the tunnel and of sessions assigned to the tunnel.

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Dial User

An end-system or router attached to an on-demand PSTN or ISDN which is either the initiator or recipient of a call.

Network Access Server (NAS)

A device providing temporary, on-demand network access to users.

This access is point-to-point using PSTN or ISDN lines.

PPTP Access Concentrator (PAC)

A device attached to one or more PSTN or ISDN lines capable of PPP operation and of handling the PPTP protocol. The PAC need only implement TCP/IP to pass traffic to one or more PNSs. It may also tunnel non-IP protocols.

PPTP Network Server (PNS)

A PNS is envisioned to operate on general-purpose computing/server platforms. The PNS handles the server side of the PPTP protocol.

Since PPTP relies completely on TCP/IP and is independent of the interface hardware, the PNS may use any combination of IP

interface hardware including LAN and WAN devices.

Session

PPTP is connection-oriented. The PNS and PAC maintain state for each user that is attached to a PAC. A session is created when end-to-end PPP connection is attempted between a dial user and the PNS. The datagrams related to a session are sent over the tunnel between the PAC and PNS.

Tunnel

A tunnel is defined by a PNS-PAC pair. The tunnel protocol is defined by a modified version of GRE [1,2]. The tunnel carries PPP datagrams between the PAC and the PNS. Many sessions are multiplexed on a single tunnel. A control connection operating over TCP controls the establishment, release, and maintenance of sessions and of the tunnel itself.

1.3. Protocol Overview

There are two parallel components of PPTP: 1) a Control Connection between each PAC-PNS pair operating over TCP and 2) an IP tunnel operating between the same PAC-PNS pair which is used to transport GRE encapsulated PPP packets for user sessions between the pair.

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1.3.1. Control Connection Overview

Before PPP tunneling can occur between a PAC and PNS, a control connection must be established between them. The control connection is a standard TCP session over which PPTP call control and management information is passed. The control session is logically associated with, but separate from, the sessions being tunneled through a PPTP tunnel. For each PAC-PNS pair both a tunnel and a control connection exist. The control connection is responsible for establishment,

management, and release of sessions carried through the tunnel. It is the means by which a PNS is notified of an incoming call at an

associated PAC, as well as the means by which a PAC is instructed to place an outgoing dial call.

A control connection can be established by either the PNS or the PAC.

Following the establishment of the required TCP connection, the PNS and PAC establish the control connection using the Start-Control- Connection-Request and -Reply messages. These messages are also used to exchange information about basic operating capabilities of the PAC and PNS. Once the control connection is established, the PAC or PNS may initiate sessions by requesting outbound calls or responding to inbound requests. The control connection may communicate changes in operating characteristics of an individual user session with a Set- Link-Info message. Individual sessions may be released by either the PAC or PNS, also through Control Connection messages.

The control connection itself is maintained by keep-alive echo messages. This ensures that a connectivity failure between the PNS and the PAC can be detected in a timely manner. Other failures can be reported via the

Wan-Error-Notify message, also on the control connection.

It is intended that the control connection will also carry management related messages in the future, such as a message allowing the PNS to request the status of a given PAC; these message types have not yet been defined.

1.3.2. Tunnel Protocol Overview

PPTP requires the establishment of a tunnel for each communicating PNS-PAC pair. This tunnel is used to carry all user session PPP packets for sessions involving a given PNS-PAC pair. A key which is present in the GRE header indicates which session a particular PPP packet belongs to.

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In this manner, PPP packets are multiplexed and demultiplexed over a single tunnel between a given PNS-PAC pair. The value to use in the key field is established by the call establishment procedure which takes place on the control connection.

The GRE header also contains acknowledgment and sequencing

information that is used to perform some level of congestion-control and error detection over the tunnel. Again the control connection is used to determine rate and buffering parameters that are used to regulate the flow of PPP packets for a particular session over the tunnel. PPTP does not specify the particular algorithms to use for congestion-control and flow-control. Suggested algorithms for the determination of adaptive time-outs to recover from dropped data or acknowledgments on the tunnel are included in section 4.4 of this document.

1.4. Message Format and Protocol Extensibility

PPTP defines a set of messages sent as TCP data on the control connection between a PNS and a given PAC. The TCP session for the control connection is established by initiating a TCP connection to port 1723 [6]. The source port is assigned to any unused port number.

Each PPTP Control Connection message begins with an 8 octet fixed header portion. This fixed header contains the following: the total length of the message, the PPTP Message Type indicator, and a "Magic Cookie".

Two Control Connection message types are indicated by the PPTP Message Type field:

1 - Control Message 2 - Management Message

Management messages are currently not defined.

The Magic Cookie is always sent as the constant 0x1A2B3C4D. Its basic purpose is to allow the receiver to ensure that it is properly synchronized with the TCP data stream. It should not be used as a means for resynchronizing the TCP data stream in the event that a transmitter issues an improperly formatted message. Loss of synchronization must result in immediate closing of the control connection’s TCP session.

For clarity, all Control Connection message templates in the next section include the entire PPTP Control Connection message header.

Numbers preceded by 0x are hexadecimal values.

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The currently defined Control Messages, grouped by function, are:

Control Message Message Code (Control Connection Management)

Start-Control-Connection-Request 1

Start-Control-Connection-Reply 2

Stop-Control-Connection-Request 3

Stop-Control-Connection-Reply 4

Echo-Request 5

Echo-Reply 6

(Call Management) Outgoing-Call-Request 7

Outgoing-Call-Reply 8

Incoming-Call-Request 9

Incoming-Call-Reply 10

Incoming-Call-Connected 11

Call-Clear-Request 12

Call-Disconnect-Notify 13

(Error Reporting) WAN-Error-Notify 14

(PPP Session Control) Set-Link-Info 15

The Start-Control-Connection-Request and -Reply messages determine which version of the Control Connection protocol will be used. The version number field carried in these messages consists of a version number in the high octet and a revision number in the low octet.

Version handling is described in section 2. The current value of the version number field is 0x0100 for version 1, revision 0.

The use of the GRE-like header for the encapsulation of PPP user packets is specified in section 4.1.

The MTU for the user data packets encapsulated in GRE is 1532 octets, not including the IP and GRE headers.

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2. Control Connection Protocol Specification

Control Connection messages are used to establish and clear user sessions. The first set of Control Connection messages are used to maintain the control connection itself. The control connection is initiated by either the PNS or PAC after they establish the

underlying TCP connection. The procedure and configuration information required to determine which TCP connections are established is not covered by this protocol.

The following Control Connection messages are all sent as user data on the established TCP connection between a given PNS-PAC pair. Note that care has been taken to ensure that all word (2 octet) and

longword (4 octet) values begin on appropriate boundaries. All data is sent in network order (high order octets first). Any "reserved"

fields MUST be sent as 0 values to allow for protocol extensibility.

2.1. Start-Control-Connection-Request

The Start-Control-Connection-Request is a PPTP control message used to establish the control connection between a PNS and a PAC. Each PNS-PAC pair requires a dedicated control connection to be

established. A control connection must be established before any other PPTP messages can be issued. The establishment of the control connection can be initiated by either the PNS or PAC. A procedure which handles the occurrence of a collision between PNS and PAC Start-Control-Connection-Requests is described in section 3.1.3.

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0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Length | PPTP Message Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Control Message Type | Reserved0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Protocol Version | Reserved1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Framing Capabilities | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Bearer Capabilities | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Maximum Channels | Firmware Revision | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| | + Host Name (64 octets) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| | + Vendor String (64 octets) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Length Total length in octets of this PPTP message, including the entire PPTP header.

PPTP Message Type 1 for Control Message.

Magic Cookie 0x1A2B3C4D. This constant value is used as a sanity check on received messages (see section 1.4).

Control Message Type 1 for Start-Control-Connection-Request.

Reserved0 This field MUST be 0.

Protocol Version The version of the PPTP protocol that the sender wishes to use.

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Framing Capabilities A set of bits indicating the type of framing that the sender of this message can provide.

The currently defined bit settings are:

1 - Asynchronous Framing supported 2 - Synchronous Framing supported Bearer Capabilities A set of bits indicating the bearer

capabilities that the sender of this message can provide. The currently defined bit settings are:

1 - Analog access supported 2 - Digital access supported

Maximum Channels The total number of individual PPP sessions this PAC can support. In Start-Control- Connection-Requests issued by the PNS, this value SHOULD be set to 0. It MUST be

ignored by the PAC.

Firmware Revision This field contains the firmware revision number of the issuing PAC, when issued by the PAC, or the version of the PNS PPTP driver if issued by the PNS.

Host Name A 64 octet field containing the DNS name of the issuing PAC or PNS. If less than 64 octets in length, the remainder of this field SHOULD be filled with octets of value 0.

Vendor Name A 64 octet field containing a vendor

specific string describing the type of PAC being used, or the type of PNS software being used if this request is issued by the PNS. If less than 64 octets in length, the remainder of this field SHOULD be filled with octets of value 0.

2.2. Start-Control-Connection-Reply

The Start-Control-Connection-Reply is a PPTP control message sent in reply to a received Start-Control-Connection-Request message. This message contains a result code indicating the result of the control connection establishment attempt.

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0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Protocol Version | Result Code | Error Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Framing Capability | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Bearer Capability | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Maximum Channels | Firmware Revision | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| | + Host Name (64 octets) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| | + Vendor String (64 octets) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Magic Cookie 0x1A2B3C4D.

Control Message Type 2 for Start-Control-Connection-Reply.

Protocol Version The version of the PPTP protocol that the sender wishes to use.

Result Code Indicates the result of the command channel establishment attempt. Current valid Result Code values are:

1 - Successful channel establishment 2 - General error -- Error Code indicates the problem

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3 - Command channel already exists;

4 - Requester is not authorized to establish a command channel 5 - The protocol version of the requester is not supported Error Code This field is set to 0 unless a "General Error" exists, in which case Result Code is set to 2 and this field is set to the value corresponding to the general error condition as specified in section 2.2.

Framing Capabilities A set of bits indicating the type of framing that the sender of this message can provide.

The currently defined bit settings are:

1 - Asynchronous Framing supported 2 - Synchronous Framing supported.

Bearer Capabilities A set of bits indicating the bearer

capabilities that the sender of this message can provide. The currently defined bit settings are:

1 - Analog access supported 2 - Digital access supported

Maximum Channels The total number of individual PPP sessions this PAC can support. In a Start-Control- Connection-Reply issued by the PNS, this value SHOULD be set to 0 and it must be ignored by the PAC. The PNS MUST NOT use this value to try to track the remaining number of PPP sessions that the PAC will allow.

Firmware Revision This field contains the firmware revision number of the issuing PAC, or the version of the PNS PPTP driver if issued by the PNS.

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Host Name A 64 octet field containing the DNS name of the issuing PAC or PNS. If less than 64 octets in length, the remainder of this field SHOULD be filled with octets of value 0.

Vendor Name A 64 octet field containing a vendor

specific string describing the type of PAC being used, or the type of PNS software being used if this request is issued by the PNS. If less than 64 octets in length, the remainder of this field SHOULD be filled with octets of value 0.

2.3. Stop-Control-Connection-Request

The Stop-Control-Connection-Request is a PPTP control message sent by one peer of a PAC-PNS control connection to inform the other peer that the control connection should be closed. In addition to closing the control connection, all active user calls are implicitly cleared.

The reason for issuing this request is indicated in the Reason field.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Reason | Reserved1 | Reserved2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Message Type 3 for Stop-Control-Connection-Request.

Reason Indicates the reason for the control connection being closed. Current valid Reason values are:

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1 (None) - General request to clear control connection

2 (Stop-Protocol) - Can’t support peer’s version of the protocol

3 (Stop-Local-Shutdown) - Requester is being shut down

Reserved1, Reserved2 These fields MUST be 0.

2.4. Stop-Control-Connection-Reply

The Stop-Control-Connection-Reply is a PPTP control message sent by one peer of a PAC-PNS control connection upon receipt of a Stop- Control-Connection-Request from the other peer.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Result Code | Error Code | Reserved1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Message Type 4 for Stop-Control-Connection-Reply.

Result Code Indicates the result of the attempt to close the control connection. Current valid Result Code values are:

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1 (OK) - Control connection closed 2 (General Error) - Control connection not closed for reason indicated in Error Code

Error Code This field is set to 0 unless a "General Error" exists, in which case Result Code is set to 2 and this field is set to the value corresponding to the general error condition as specified in section 2.2.

2.5. Echo-Request

The Echo-Request is a PPTP control message sent by either peer of a PAC-PNS control connection. This control message is used as a "keep- alive" for the control connection. The receiving peer issues an Echo-Reply to each Echo-Request received. As specified in section 3.1.4, if the sender does not receive an Echo-Reply in response to an Echo-Request, it will eventually clear the control connection.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Message Type 5 for Echo-Request.

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Identifier A value set by the sender of the Echo-

Request that is used to match the reply with the corresponding request.

2.6. Echo-Reply

The Echo-Reply is a PPTP control message sent by either peer of a PAC-PNS control connection in response to the receipt of an Echo- Request.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Result Code | Error Code | Reserved1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Message Type 6 for Echo-Reply.

Identifier The contents of the identify field from the received Echo-Request is copied to this field.

Result Code Indicates the result of the receipt of the Echo-Request. Current valid Result Code values are:

1 (OK) - The Echo-Reply is valid 2 (General Error) - Echo-Request not accepted for the reason indicated in Error Code

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Error Code This field is set to 0 unless a "General Error" condition exists, in which case Result Code is set to 2 and this field is set to the value corresponding to the general error condition as specified in section 2.2.

2.7. Outgoing-Call-Request

The Outgoing-Call-Request is a PPTP control message sent by the PNS to the PAC to indicate that an outbound call from the PAC is to be established. This request provides the PAC with information required to make the call. It also provides information to the PAC that is used to regulate the transmission of data to the PNS for this session once it is established.

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0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Call ID | Call Serial Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Minimum BPS | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Maximum BPS | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Bearer Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Framing Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Packet Recv. Window Size | Packet Processing Delay | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Phone Number Length | Reserved1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| | + Phone Number (64 octets) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| | + Subaddress (64 octets) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Message Type 7 for Outgoing-Call-Request.

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Call ID A unique identifier, unique to a particular PAC-PNS pair assigned by the PNS to this session. It is used to multiplex and demultiplex data sent over the tunnel between the PNS and PAC involved in this session.

Call Serial Number An identifier assigned by the PNS to this session for the purpose of identifying this particular session in logged session

information. Unlike the Call ID, both the PNS and PAC associate the same Call Serial Number with a given session. The combination of IP address and call serial number SHOULD be unique.

Minimum BPS The lowest acceptable line speed (in bits/second) for this session.

Maximum BPS The highest acceptable line speed (in bits/second) for this session.

Bearer Type A value indicating the bearer capability required for this outgoing call. The currently defined values are:

1 - Call to be placed on an analog channel

2 - Call to be placed on a digital channel

3 - Call can be placed on any type of channel

Framing Type A value indicating the type of PPP framing to be used for this outgoing call.

1 - Call to use Asynchronous framing 2 - Call to use Synchronous framing 3 - Call can use either type of framing

Packet Recv. Window Size The number of received data packets the PNS will buffer for this session.

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Packet Processing Delay A measure of the packet processing delay that might be imposed on data sent to the PNS from the PAC. This value is specified in units of 1/10 seconds. For the PNS this number should be very small. See section 4.4 for a description of how this value is determined and used.

Phone Number Length The actual number of valid digits in the Phone Number field.

Phone Number The number to be dialed to establish the outgoing session. For ISDN and analog calls this field is an ASCII string. If the Phone Number is less than 64 octets in length, the remainder of this field is filled with

octets of value 0.

Subaddress A 64 octet field used to specify additional dialing information. If the subaddress is less than 64 octets long, the remainder of this field is filled with octets of value 0.

2.8. Outgoing-Call-Reply

The Outgoing-Call-Reply is a PPTP control message sent by the PAC to the PNS in response to a received Outgoing-Call-Request message. The reply indicates the result of the outgoing call attempt. It also provides information to the PNS about particular parameters used for the call. It provides information to allow the PNS to regulate the transmission of data to the PAC for this session.

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0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Call ID | Peer’s Call ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Result Code | Error Code | Cause Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Connect Speed | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Packet Recv. Window Size | Packet Processing Delay | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Physical Channel ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Message Type 8 for Outgoing-Call-Reply.

Call ID A unique identifier for the tunnel, assigned by the PAC to this session. It is used to multiplex and demultiplex data sent over the tunnel between the PNS and PAC involved in this session.

Peer’s Call ID This field is set to the value received in the Call ID field of the corresponding Outgoing-Call-Request message. It is used by the PNS to match the Outgoing-Call-Reply with the Outgoing-Call-Request it issued. It also is used as the value sent in the GRE header for mux/demuxing.

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Result Code This value indicates the result of the Outgoing-Call-Request attempt. Currently valid values are:

1 (Connected) - Call established with no errors

2 (General Error) - Outgoing Call not established for the reason indicated in Error Code

3 (No Carrier) - Outgoing Call failed due to no carrier detected

4 (Busy) - Outgoing Call failed due to detection of a busy signal

5 (No Dial Tone) - Outgoing Call failed due to lack of a dial tone 6 (Time-out) - Outgoing Call was not established within time allotted by PAC

7 (Do Not Accept) - Outgoing Call administratively prohibited Error Code This field is set to 0 unless a "General Error" condition exists, in which case Result Code is set to 2 and this field is set to the value corresponding to the general error condition as specified in section 2.2.

Cause Code This field gives additional failure

information. Its value can vary depending upon the type of call attempted. For ISDN call attempts it is the Q.931 cause code.

Connect Speed The actual connection speed used, in bits/second.

Packet Recv. Window Size The number of received data packets the PAC will buffer for this session.

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Packet Processing Delay A measure of the packet processing delay that might be imposed on data sent to the PAC from the PNS. This value is specified in units of 1/10 seconds. For the PAC, this number is related to the size of the buffer used to hold packets to be sent to the client and to the speed of the link to the client. This value should be set to the maximum delay that can normally occur

between the time a packet arrives at the PAC and is delivered to the client. See section 4.4 for an example of how this value is determined and used.

Physical Channel ID This field is set by the PAC in a vendor- specific manner to the physical channel number used to place this call. It is used for logging purposes only.

2.9. Incoming-Call-Request

The Incoming-Call-Request is a PPTP control message sent by the PAC to the PNS to indicate that an inbound call is to be established from the PAC. This request provides the PNS with parameter information for the incoming call.

This message is the first in the "three-way handshake" used by PPTP for establishing incoming calls. The PAC may defer answering the call until it has received an Incoming-Call-Reply from the PNS

indicating that the call should be established. This mechanism allows the PNS to obtain sufficient information about the call before it is answered to determine whether the call should be answered or not.

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0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Call ID | Call Serial Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Call Bearer Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Physical Channel ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Dialed Number Length | Dialing Number Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| | + Dialed Number (64 octets) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| | + Dialing Number (64 octets) + |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| | + Subaddress (64 octets) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Message Type 9 for Incoming-Call-Request.

Call ID A unique identifier for this tunnel,

assigned by the PAC to this session. It is used to multiplex and demultiplex data sent over the tunnel between the PNS and PAC involved in this session.

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Call Serial Number An identifier assigned by the PAC to this session for the purpose of identifying this particular session in logged session

information. Unlike the Call ID, both the PNS and PAC associate the same Call Serial Number to a given session. The combination of IP address and call serial number should be unique.

Bearer Type A value indicating the bearer capability used for this incoming call. Currently defined values are:

1 - Call is on an analog channel 2 - Call is on a digital channel Physical Channel ID This field is set by the PAC in a vendor- specific manner to the number of the physical channel this call arrived on.

Dialed Number Length The actual number of valid digits in the Dialed Number field.

Dialing Number Length The actual number of valid digits in the Dialing Number field.

Dialed Number The number that was dialed by the caller.

For ISDN and analog calls this field is an ASCII string. If the Dialed Number is less than 64 octets in length, the remainder of this field is filled with octets of value 0.

Dialing Number The number from which the call was placed.

For ISDN and analog calls this field is an ASCII string. If the Dialing Number is less than 64 octets in length, the remainder of this field is filled with octets of value 0.

Subaddress A 64 octet field used to specify additional dialing information. If the subaddress is less than 64 octets long, the remainder of this field is filled with octets of value 0.

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2.10. Incoming-Call-Reply

The Incoming-Call-Reply is a PPTP control message sent by the PNS to the PAC in response to a received Incoming-Call-Request message. The reply indicates the result of the incoming call attempt. It also provides information to allow the PAC to regulate the transmission of data to the PNS for this session.

This message is the second in the three-way handshake used by PPTP for establishing incoming calls. It indicates to the PAC whether the call should be answered or not.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Call ID | Peer’s Call ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Result Code | Error Code | Packet Recv. Window Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Packet Transmit Delay | Reserved1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Message Type 10 for Incoming-Call-Reply.

Call ID A unique identifier for this tunnel assigned by the PNS to this session. It is used to multiplex and demultiplex data sent over the tunnel between the PNS and PAC involved in this session.

Peer’s Call ID This field is set to the value received in the Call ID field of the corresponding

Incoming-Call-Request message. It is used by

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the PAC to match the Incoming-Call-Reply with the Incoming-Call-Request it issued.

This value is included in the GRE header of transmitted data packets for this session.

Result Code This value indicates the result of the Incoming-Call-Request attempt. Current valid Result Code values are:

1 (Connect) - The PAC should answer the incoming call

2 (General Error) - The Incoming Call should not be established due to the reason indicated in Error Code

3 (Do Not Accept) - The PAC should not accept the incoming call. It should hang up or issue a busy indication Error Code This field is set to 0 unless a "General Error" condition exists, in which case Result Code is set to 2 and this field is set to the value corresponding to the general error condition as specified in section 2.2.

Packet Transmit Delay A measure of the packet processing delay that might be imposed on data sent to the PAC from the PNS. This value is specified in units of 1/10 seconds.

2.11. Incoming-Call-Connected

The Incoming-Call-Connected message is a PPTP control message sent by the PAC to the PNS in response to a received Incoming-Call-Reply. It provides information to the PNS about particular parameters used for the call. It also provides information to allow the PNS to regulate the transmission of data to the PAC for this session.

This message is the third in the three-way handshake used by PPTP for establishing incoming calls. It provides a mechanism for providing the PNS with additional information about the call that cannot, in

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general, be obtained at the time the Incoming-Call-Request is issued by the PAC.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Peer’s Call ID | Reserved1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Connect Speed | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Packet Recv. Window Size | Packet Transmit Delay | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Framing Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Message Type 11 for Incoming-Call-Connected.

Peer’s Call ID This field is set to the value received in the Call ID field of the corresponding Incoming-Call-Reply message. It is used by the PNS to match the Incoming-Call-Connected with the Incoming-Call-Reply it issued.

Connect Speed The actual connection speed used, in bits/second.

Packet Transmit Delay A measure of the packet processing delay that might be imposed on data sent to the PAC from the PNS. This value is specified in units of 1/10 seconds.

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Framing Type A value indicating the type of PPP framing being used by this incoming call.

1 - Call uses asynchronous framing 2 - Call uses synchronous framing 2.12. Call-Clear-Request

The Call-Clear-Request is a PPTP control message sent by the PNS to the PAC indicating that a particular call is to be disconnected. The call being cleared can be either an incoming or outgoing call, in any state. The PAC responds to this message with a Call-Disconnect- Notify message.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Call ID | Reserved1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Message Type 12 for Call-Clear-Request.

Call ID The Call ID assigned by the PNS to this call. This value is used instead of the Peer’s Call ID because the latter may not be known to the PNS if the call must be aborted during call establishment.

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2.13. Call-Disconnect-Notify

The Call-Disconnect-Notify message is a PPTP control message sent by the PAC to the PNS. It is issued whenever a call is disconnected, due to the receipt by the PAC of a Call-Clear-Request or for any other reason. Its purpose is to inform the PNS of both the disconnection and the reason for it.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Call ID | Result Code | Error Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Cause Code | Reserved1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| | + Call Statistics (128 octets) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Message Type 13 for Call-Disconnect-Notify.

Call ID The value of the Call ID assigned by the PAC to this call. This value is used instead of the Peer’s Call ID because the latter may not be known to the PNS if the call must be aborted during call establishment.

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Result Code This value indicates the reason for the disconnect. Current valid Result Code values are:

1 (Lost Carrier) - Call disconnected due to loss of carrier

2 (General Error) - Call disconnected for the reason indicated in Error Code

3 (Admin Shutdown) - Call disconnected for administrative reasons

4 (Request) - Call disconnected due to received Call-Clear-Request

Error Code This field is set to 0 unless a "General Error" condition exists, in which case the Result Code is set to 2 and this field is set to the value corresponding to the general error condition as specified in section 2.2.

Cause Code This field gives additional disconnect information. Its value varies depending on the type of call being disconnected. For ISDN calls it is the Q.931 cause code.

Call Statistics This field is an ASCII string containing vendor-specific call statistics that can be logged for diagnostic purposes. If the length of the string is less than 128, the remainder of the field is filled with octets of value 0.

2.14. WAN-Error-Notify

The WAN-Error-Notify message is a PPTP control message sent by the PAC to the PNS to indicate WAN error conditions (conditions that occur on the interface supporting PPP). The counters in this message are cumulative. This message should only be sent when an error

occurs, and not more than once every 60 seconds. The counters are reset when a new call is established.

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0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Control Message type | Reserved0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| CRC Errors | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Framing Errors | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Hardware Overruns | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Buffer Overruns | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Time-out Errors | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Alignment Errors | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Message Type 14 for WAN-Error-Notify.

Peer’s Call ID Th Call ID assigned by the PNS to this call.

CRC Errors Number of PPP frames received with CRC errors since session was established.

Framing Errors Number of improperly framed PPP packets received.

Hardware Overruns Number of receive buffer over-runs since session was established.

Buffer Overruns Number of buffer over-runs detected since session was established.

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Time-out Errors Number of time-outs since call was established.

Alignment Errors Number of alignment errors since call was established.

2.15. Set-Link-Info

The Set-Link-Info message is a PPTP control message sent by the PNS to the PAC to set PPP-negotiated options. Because these options can change at any time during the life of the call, the PAC must be able to update its internal call information dynamically and perform PPP negotiation on an active PPP session.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Magic Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Control Message type | Reserved0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Send ACCM | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Receive ACCM | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Message Type 15 for Set-Link-Info.

Peer’s Call ID The value of the Call ID assigned by the PAC to this call.

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Send ACCM The send ACCM value the client should use to process outgoing PPP packets. The default value used by the client until this message is received is 0XFFFFFFFF. See [7].

Receive ACCM The receive ACCM value the client should use to process incoming PPP packets. The default value used by the client until this message is received is 0XFFFFFFFF. See [7].

2.16. General Error Codes

General error codes pertain to types of errors which are not specific to any particular PPTP request, but rather to protocol or message format errors. If a PPTP reply indicates in its Result Code that a general error occurred, the General Error value should be examined to determined what the error was. The currently defined General Error codes and their meanings are:

0 (None) - No general error

1 (Not-Connected) - No control connection exists yet for this PAC-PNS pair

2 (Bad-Format) - Length is wrong or Magic Cookie value is incorrect

3 (Bad-Value) - One of the field values was out of range or reserved field was non-zero

4 (No-Resource) - Insufficient resources to handle this command now

5 (Bad-Call ID) - The Call ID is invalid in this context 6 (PAC-Error) - A generic vendor-specific error occurred in the PAC

3. Control Connection Protocol Operation

This section describes the operation of various PPTP control

connection functions and the Control Connection messages which are used to support them. The protocol operation of the control

connection is simplified because TCP is used to provide a reliable transport mechanism. Ordering and retransmission of messages is not a concern at this level. The TCP connection itself, however, can close at any time and an appropriate error recovery mechanism must be provided to handle this case.

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Some error recovery procedures are common to all states of the control connection. If an expected reply does not arrive within 60 seconds, the control connection is closed, unless otherwise

specified. Appropriate logging should be implemented for easy

determination of the problems and the reasons for closing the control connection.

Receipt of an invalid or malformed Control Connection message should be logged appropriately, and the control connection should be closed and restarted to ensure recovery into a known state.

3.1. Control Connection States

The control connection relies on a standard TCP connection for its service. The PPTP control connection protocol is not distinguishable between the PNS and PAC, but is distinguishable between the

originator and receiver. The originating peer is the one which first attempts the TCP open. Since either PAC or PNS may originate a

connection, it is possible for a TCP collision to occur. See section 3.1.3 for a description of this situation.

3.1.1. Control Connection Originator (may be PAC or PNS) TCP Open Indication

/Send Start Control

Connection Request +---+

+--->| wait_ctl_reply | | +---+

^ V | V

+---+ Receive Start Ctl | +---+

^ | V Local Terminate | Receive Stop Control | | /Send Stop

| Connection Request | | Control Request | /Send Stop Control Reply V V

| Close TCP +---+

+---| wait_stop_reply | +---+

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idle

The control connection originator attempts to open a TCP

connection to the peer during idle state. When the TCP connection is open, the originator transmits a send Start-Control-

Connection-Request and then enters the wait_ctl_reply state.

wait_ctl_reply

The originator checks to see if another TCP connection has been requested from the same peer, and if so, handles the collision situation described in section 3.1.3.

When a Start-Control-Connection-Reply is received, it is examined for a compatible version. If the version of the reply is lower than the version sent in the request, the older (lower) version should be used provided it is supported. If the version in the reply is earlier and supported, the originator moves to the

established state. If the version is earlier and not supported, a Stop-Control-Connection-Request SHOULD be sent to the peer and the originator moves into the wait_stop_reply state.

established

An established connection may be terminated by either a local condition or the receipt of a Stop-Control-Connection-Request. In the event of a local termination, the originator MUST send a Stop-Control-Connection-Request and enter the wait_stop_reply state.

If the originator receives a Stop-Control-Connection-Request it SHOULD send a Stop-Control-Connection-Reply and close the TCP connection making sure that the final TCP information has been "pushed" properly.

wait_stop_reply

If a Stop-Control-Connection-Reply is received, the TCP connection SHOULD be closed and the control connection becomes idle.